1. Technical Field
The present invention relates to an optical scanning device and an image forming apparatus, and more particularly relates to an optical scanning device at which plural optical systems and a heat-generating component are disposed in a single container, the optical systems respectively guiding plural light beams which have been deflectingly scanned by a light-deflecting element, and to an image forming apparatus which is equipped with this optical scanning device.
2. Related Art
An electrophotographic-system color image forming apparatus deflectingly scans plural light beams corresponding to the colors Y (yellow), M (magenta), C (cyan) and K (black), or the like, with an optical deflector which is mounted at an optical scanning device, and forms a color image by focusing the respective colors through plural optical systems onto a photosensitive drum. In such a color image forming apparatus, image formation timings of the colors are regulated in accordance with a device temperature, which is measured by an environment sensor (a temperature sensor). Thus, color shifts between color images (reading registration errors) that are caused by temperature variations of the device are corrected for (“color registration correction”).
In recent years, in order to suppress costs, housings of optical scanning devices have come to be made of molded resin components, and as optical deflectors, inexpensive general purpose unitized components formed as units are being used. In such units, a polygon mirror and a motor are disposed on a circuit board, which serves as a base for the optical deflector, and a motor-driving IC, for controlling rotary driving of the motor, and the like are also mounted at the circuit board.
However, with an optical deflector which is formed as a unit in this manner, because the whole of the optical deflector is accommodated in the housing of the optical scanning device, heat which is generated by heat-generating components, such as the motor-driving IC and the like, tends to accumulate within the housing. Hence, with a housing made of resin, which has lower thermal conductivity (heat absorption and heat dissipation characteristics) than a metal model made of die-cast aluminum or the like, interior heat is less easily propagated through the housing and dissipated. Therefore, particularly just after the device starts to operate, when the amount of temperature increase is large, there is a difference in temperature gradient between an interior temperature of the optical scanning device (the housing) and the temperature that is measured by an environment sensor. Thus, there is a problem in that color registration errors will occur.
As is shown in FIG. 14, when, for example, the temperature variation of a color image forming apparatus is observed over a 30-minute period after startup, the motor-driving IC of the optical deflector rapidly rises in temperature for about 3 minutes after startup, and then gradually stabilizes. Meanwhile, the interior of the optical scanning device (housing) gradually rises in temperature for about 25 minutes after startup, and substantially stabilizes at an increase of about 3.5° C.
On the other hand, because propagation of heat through the housing is low and propagation of heat through the air is dominant after startup, a rate of heat conduction to the environment sensor is slow, and the environment sensor has no observable rise in temperature for about 8 minutes after startup, thereafter rises only gradually, and does not match the temperature in the optical scanning device until about 30 minutes has passed.
Thus, just after the device starts operation, there is a difference between a gradient of the temperature in the optical scanning device and a gradient of the device temperature that is measured by the environment sensor, and the rise of the environment sensor is slower than the temperature rise of the optical scanning device interior. Therefore, when a reading registration difference between, for example, the color C and the color K is observed, as is shown in the graph for an IOT (Image Output Terminal) in FIG. 15, a registration error just before input of a registration control cycle is large. Furthermore, when the polygon mirror of the optical scanning device (ROS: Raster Output Scanner) is rotated and laser light sources are illuminated, a graph showing the reading registration difference between the color C and the color K at the optical scanning device (‘ROS unit body’) is similar to the above-mentioned graph for the IOT. From this, it is understood that effects of heat sources other than the optical scanning device on deterioration in color registration at the IOT just after startup are small, and the deterioration in color registration is mainly determined by characteristics of the optical scanning device.
Further, as shown in FIG. 16, reading registration offsets of the color C and the color K are set in relatively opposite directions, with the color C at a minus side, and the color K at a plus side. Thus, offset amounts are large. Note that differences between offset amounts of the color C and offset amounts of the color K in FIG. 16 constitute the graph of reading registration errors of the ROS unit body shown in FIG. 15.
FIG. 17 shows a schematic diagram of the structure of the optical scanning device at which the various data shown in FIGS. 14 to 16 have been measured. At an optical scanning device 110CK shown in FIG. 17, two different optical systems corresponding to the color C and the color K are provided at a single housing (optical casing) 112, which is made of resin. A light beam K corresponding to the color K, which is deflectingly scanned by a polygon mirror 54 of an optical deflector, passes through f-θ lenses 56 and 58, is reflected by a total of four mirrors—a cylindrical mirror 60K, a reflection mirror 62K, a cylindrical mirror 64K and a reflection mirror 66K—and is focused on a photosensitive drum 24K. Similarly, a light beam C corresponding to the color C, which is deflectingly scanned by the polygon mirror 54, passes through the f-θ lenses 56 and 58, is reflected by a total of three mirrors—a cylindrical mirror 60C, a reflection mirror 62C and a cylindrical mirror 64C—and is focused on a photosensitive drum 24C.
Now, just after startup of the device, besides the motor-driving IC of the optical deflector, a driving IC at which a laser light source driver (LDD) or the like is mounted also rapidly rises in temperature at the time of startup. Air inside the optical scanning device 110CK is warmed by these heat-generating components, and the air is agitated by rotation of the polygon mirror 54. Consequently, a distribution of temperature in the optical scanning device 110CK alters or a hot air flow impinges on the optical system (for example, on a reflection mirror directly or on a support of a reflection mirror), and a temperature thereof is increased. Thus, when, for example, the light beams C and K pass through the f-θ lenses 56 and 58 and are initially incident on the cylindrical mirrors 60C and 60K and are inclined in the same direction, the light beams C and K that have been reflected by the cylindrical mirrors 60C and 60K are shifted as shown by the broken lines, and the reading registrations on the photosensitive drums 24C and 24K are offset to respectively opposite sides (see FIG. 16). Thus, the difference which is a color registration error becomes large.
As countermeasures for the color registration error which is generated in this manner, for example, reducing a time interval between temperature measurements by the environment sensor and increasing a number of registration control cycles have been considered. However, in such cases, while the color registration error described above can be avoided, the number of down-times, at which image output operations are stopped, increases and usability deteriorates.